That chart is a classic case of "extremely truncated bottom" (that I learned in school with the help of the book "How to Lie with Statistics"). If you look closely, it occurs 688 times over 400 years as opposed to 684 times for Saturday the 13th, a difference of 4 times, or ONCE every hundred years, also known as "statistically irrelevant". And remember kids, Irrelevant Never Forgets.